1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gas, and, more particularly, to a catalyst suitable for purifying oxides of nitrogen (NOx) in exhaust gas from an engine of a motor vehicle.
2. Description of Related Art
Recently, much attentions have been focused on lean-burn engines for motor vehicles which have a low fuel consumption. Such a lean-burn engine, however, generates more oxides of nitrogen (Nox) because a lean fuel mixture is burned in an excessive oxygen. Thus, there is still a demand for a catalyst suitable for purifying nitrogen oxides (Nox) more efficiently.
An example of such a catalyst, as disclosed in Japanese Unexamined Patent Publication No.7-108172, is the one that has two layers supported on a carrier, of which an base or base layer contains alumina on which alkali earth metal, such as barium, is supported and of which an over or over layer contains alumina on which platinum and rhodium are supported.
As described in Japanese Unexamined Patent Publication No.5-68888, there has been known in the art a catalyst comprising two layers coated on a carrier, one of which, namely an base layer, bearing .gamma.-alumina supporting a precious metal or metals and another of which, namely an over layer, bearing zeolite supporting copper. Another example is a catalyst such as described in, for example, Japanese Unexamined Patent Publication No.7-24333, which comprises two layers, i.e. an base layer bearing platinum, zeolite supporting rhodium and platinum, and cerium, and an over layer bearing platinum, zeolite supporting rhodium and iridium, and cerium (or otherwise alumina).
In using a catalyst which comprises two catalyst layers on a carrier of which an base layer has a precious metal as an activated species to purify oxides of nitrogen (Nox) such as described in the above mentioned Japanese Unexamined Patent Publication No.5-68888 or No.7-24333, it is necessary to disperse the precious metal into the base layer as desired. As a practical matter, it is, however, difficult to disperse as desired, due to which a drastic improvement of NOx purification ratio is hardly obtainable. In particular, in the case that the catalyst has been exposed heat and aged, or an exhaust gas is hard to flow through the catalyst layers in an engine operation range of high engine speeds and high engine loads.